Fossil Bryozoans

color photo of fan-shaped bryozoan

The Pennsylvanian Septopora exemplifies the branching form of some bryozoan colonies. This specimen is from the Kansas City Formation, Jackson County, Missouri.

Description: Bryozoans are some of the most abundant fossils in the world. They are also widespread today, both in marine and freshwater environments, living at all latitudes and at depths ranging downward to at least 27,900 feet (8,500 meters).

Marine bryozoans show up in the fossil record in the early part of the Ordovician Period, about 470 million years ago. In Kansas, fossil bryozoans are common in the Pennsylvanian and Permian rocks of the eastern part of the state.

color photo of different bryozoan fragments

Bryozoan fossils from the Topeka Limestone in Kansas. These fossils were deposited during the Pennsylvanian Period, about 300 million years ago, and illustrate the branching and netlike forms of some bryozoan colonies.

Freshwater bryozoans are virtually unknown as fossils, presumably because they did not have mineralized skeletons. Throughout their nearly 500-million-year history, marine bryozoans have been abundant and widely distributed geographically. They are the most abundant fossils in many limestones, calcareous shales, and mudstones. At least 3,500 living species and 15,000 fossil species are known.

Bryozoans are small animals (just large enough to be seen with the naked eye) that live exclusively in colonies. In fact, the Phylum Bryozoa is the only animal phylum in which all known species form colonies. The name comes from two Greek words, bryon (moss) and zoon (animal), and bryozoans are sometimes called moss animals because some colonies resemble mosses.

Bryozoans are sometimes confused with corals, another colonial group of animals. Like corals, most bryozoans secrete external skeletons made of calcium carbonate, which form the framework of the colony. Bryozoans, however, are more complex organisms than corals and generally don't build reefs.

Individual members of a bryozoan colony are called zooids. Although all zooids in the colony are physically connected, each lives in its own calcium carbonate compartment. Some or all zooids in a colony are feeding zooids, equipped with tentacle-bearing feeding organs. Bryozoans feed by projecting these tentacles into the water through openings in their external skeletons (see drawing below). The tentacles have tiny moving filaments called cilia, which create currents that draw microscopic organisms and plants into the mouth. Feeding zooids also have an alimentary canal, muscles, a nervous system and a U-shaped digestive tract. Simple egg- and sperm-producing structures are also present in some zooids in every colony.

drawing of branching bryozoan, genus Rhombopora, with pores covering surface enlarged view of regularly spaced pores or openings

Close-up drawing of Rhombopora, showing the openings in the external skeleton, through which the individual zooids extended their tentacles for feeding. Rhombopora fossils are common in Pennsylvanian and Permian outcrops in Kansas (drawings by Al Kamb, KU Natural History Museum, Invertebrate Paleontology).

Bryozoan colonies start out with a single zooid, which may be produced either sexually or asexually (through budding) by the parent colony. As this original zooid begins feeding, it buds to form additional genetically identical zooids. These new zooids also bud, forming the colony. Large colonies may consist of hundreds of thousands or even millions of zooids.

Some bryozoans built colonies that grew from the seafloor in branching structures; these fossils look like something like twigs. Other species erected netlike frameworks, while some spread like a crust on shells, rocks, plants, and even other bryozoan colonies. Almost all the fossils are fragments of colonies; only rarely is an entire colony preserved.

drawing of encrusting bryozoan, looks like a lumpy blob with covered with pores  

The colonies of Tabulipora, a bryozoan found in Kansas, sometimes were sheetlike (as shown here) and are sometimes found on other fossils. Tabulipora is known from the Mississippian to the Permian.

enlarged drawing of netlike bryozoan showing framework structure covered with pores  

Fenestella, another bryozoan found in Kansas rocks, is one of the bryozoans whose colonies had a netlike structure. This fossil genus is also known from the Mississippian to the Permian (drawings by Al Kamb, KU Natural History Museum, Invertebrate Paleontology).

Bryozoan fossils turn up frequently in the Pennsylvanian and Permian rocks of eastern Kansas. The Florena Shale in Riley and Pottawatomie counties is an excellent place to find bryozoans, and they are also common in the Plattsmouth Limestone Member (of the Oread Limestone), the Beil Limestone Member (of the Lecompton Limestone), and the Topeka Limestone. Bryozoans are less common in the Cretaceous rocks to the west.

Stratigraphic Range: Lower Ordovician to Holocene.

Taxonomic Classification: Bryozoans belong to Kingdom Animalia, Phylum Bryozoa. The phylum is divided into three classes, the Phylactolaemata (freshwater bryozoans), the Stenolaemata, and Gymnolaemata.

Sources

Boardman, R. S., Cheetham, A. H., and Blake, D. B., Introduction to the Bryozoa; in, Treatise on Invertebrate Paleontology, Bryozoa, Revised (Introduction, Order Cystoporata, Order Cryptostomata) , Part G, Vol. 1, R. A. Robison, ed.: Boulder, Colorado and Lawrence, Kansas, Geological Society of America and The University of Kansas, p. 3-48.

Boardman, Richard S., and Cheetham, Alan H., 1987, Phylum Bryozoa; in, Fossil Invertebrates, R. S. Boardman, A. H. Cheetham, and A. J. Rowell, eds.: Boston, Blackwell Scientific Publications, p. 497-549.

Clarkson, E. N. K., 1979, Invertebrate Palaeontology and Evolution, 3rd Edition: London, Chapman and Hall, 434 p.

McKinney, Frank K., 1996, Bryozoans: The Paleontological Society, pamphlet, unnumbered sheet.

Moore, Raymond C., Lalicker, Cecil G., and Fischer, Alfred G., 1952, Invertebrate Fossils: New York, McGraw-Hill Book Co., 766 p.

Text by Liz Brosius, Kansas Geological Survey. Unless noted otherwise, illustrations by Jennifer Sims, Kansas Geological Survey; photographs by John Charlton, Kansas Geological Survey.

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